Micas are composed of layers. Each mica layer may be visualized as a sandwich comprising two outer layers of silicon tetrahedra and an inner layer of aluminum octahedra (i.e., 2:1 layered mica). These micas are generally represented by the general formula: EQU A.sub.x [M.sub.2-3 T.sub.4 O.sub.10 (OH).sub.2 ]
where M designates the octahedral cation, T designates the tetrahedral cation, and A designates the exchangeable interlayer cations. The T coordinated ion is commonly Si.sup.+4, Al.sup.+3, or Fe.sup.+3, but could also include several other four-coordinate ions, e.g., P.sup.+5, B.sup.+3, Ga.sup.+3, Cr.sup.+3, Ge.sup.+4, Be.sup.+2, etc. The M coordinated ion is typically Al.sup.+3 or Mg.sup.+2, but could also include many other possible hexacoordinate ions, e.g., Fe.sup.+3, Fe.sup.+2, Ni.sup.+2, Co.sup.+2, Li.sup.+, Cr.sup.+3, V.sup.+3, etc. Mg.sup.+2 is preferred in this invention.
The preparation of pillared tetrasilicic micas for use as catalyst supports is disclosed in a paper by J. W. Johnson and John F. Brody, "Pillared Clays and Micas," Materials Research Society Symposium Proceedings 111, 257 (1988). A method of catalytic cracking, isomerizing or reforming hydrocarbons employing pillared fluorotetrasilicic micas is disclosed in European patent Application No. 0341023A2. The preparation of transition-metal pillared micas which are prepared from transition metal-pillaring solutions is described in U.S. Pat. No. 4,666,877. Uses of Pt-pillared clays, or micas, are described in V. N. Parulekar and J. W. Hightower, "Hydroisomerization of n-Paraffins on a Platinum-Rhenium/Pillared Clay Mineral catalyst," Applied Catalysis 35, 249-262 (1987) and C. Doblin, J. F. Mathews, and T. W. Turney, "Hydrocracking and Isomerization of n-Octane and 2,2,4-Trimethylpentane over a Pt/Alumina-Pillared Clay," Applied Catalysis 70, 197-212 (1991). Consequently, while micas and clays are taught as being a suitable catalyst support for a variety of petrochemical reactions, pillared micas are not taught as being useful for dehydrogenation catalysts.
Transportation fuels, particularly motor gasoline, contains a relatively high level of aromatic components, such as benzene. These fuels, while relative high in octane number, are facing ever growing difficulty meeting ever stricter governmental environmental regulations with regard to emissions. This is primarily because of the high levels of aromatics. Consequently, there is much work being done to develop what has become known as "low emissions fuels". An important aspect of this work involves the substitution of non-aromatic components, having a relatively high octane value, for aromatic components of the fuel.
A class of non-aromatic components having relatively high octane value, which has been proposed for the production of low emissions fuels, are oxygenates. Non-limiting examples of preferred oxygenates for use in low emissions fuels include the unsymmetrical dialkyl ethers, particularly methyl tert-butyl ether (MTBE), ethyl tert-butyl ether (ETBE), and tert-amylmethyl ether (TAME). Conventional methods of manufacture of such ethers are typically based on liquid-phase reactions, such as the reaction of iso-butylene with methanol over cation-exchanged resins. This has created substantial demand for oxygenate building blocks, such as iso-butylene. Furthermore, other low carbon, or light, olefins are also in demand for the same reasons.
Low carbon number olefins, for example those having 2 to 10 carbon atoms, are typically obtained by the dehydrogenation of the corresponding paraffinic hydrocarbon. One method for light paraffin dehydrogenation is the so-called oxidative dehydrogenation process. In this process the light alkanes are reacted with oxygen over a suitably prepared mixed metal oxide catalyst to produce a mixture of olefin, water, CO.sub.x, and unreacted paraffin. While high conversions combined with high olefin selectivities can be achieved, such a process has a number of disadvantages including loss of fuel value due to water and CO.sub.x formation. Furthermore, process operations are relatively costly and there are industrial hazards associated with exothermic combustion reactions.
A more direct and preferred approach is direct dehydrogenation over a suitable catalyst to produce olefins and molecular hydrogen. This chemistry has recently received considerable interest, although high reaction temperatures in the range of 500.degree.-650.degree. C. are required to obtain a significant equilibrium yield (e.g., 15-65%) of olefin. Moreover, under these reaction conditions, light alkane hydrogenolysis, for example to methane, is a competing and undesirable reaction. Most catalysts studied to date have not shown very high selectivities for dehydrogenation versus hydrogenolysis, or they have suffered from rapid catalyst deactivation necessitating frequent regeneration. As a consequence, process economics have not been favorable. Consequently, incentives exist for catalysts which: have high selectivity for the production of olefins; have improved resistance to deactivation; and can be regenerated using simple procedures, such as air treatment.